This invention relates in general to plasma arc cutting and welding processes and apparatus. More specifically, it relates to a process and apparatus for dual flow piercing and cutting of metal workpieces that is faster, has a better cut quality, and protects the torch against splattered molten metal through the use of a high velocity gas secondary gas flow of well-defined flow conditions and a novel composition.
Plasma arc torches have a wide variety of applications such as the cutting of thick plates of steel and the cutting of comparatively thin sheets of galvanized metal commonly used in heating, ventilating and air conditioning (HVAC) systems. The basic components of a plasma arc torch include a torch body, an electrode (cathode) mounted within the body, a nozzle (anode) with a central exit orifice, a flow of an ionizable gas, electrical connections, passages for cooling and arc control fluids, and a power supply that produces a pilot arc in the gas, typically between the electrode and the nozzle, and then a plasma arc, a conductive flow of the ionized gas from the electrode to a workpiece. The gas can be non-oxidizing, e.g. nitrogen, argon/hydrogen, or argon, or oxidizing, e.g. oxygen or air.
Various plasma arc torches of this general type are described in U.S. Pat. Nos. 3,641,304 to Couch and Dean, 3,833,787 to Couch, 4,203,022 to Couch and Bailey, 4,421,970 to Couch, 4,791,268 to Sanders and Couch, and 4,816,637 to Sanders and Couch, all commonly assigned with the present application. Plasma arc torches and related products are sold in a variety of models by Hypertherm, Inc. of Hanover, N.H. The MAX 100 brand torch of Hypertherm is typical of the medium power torches (100 ampere output) using air as the working gas and useful for both plate fabrication and HVAC applications. The HT 400 brand torch is typical of the high power torches (260 amperes) often using oxygen as the working gas. High power torches are typically water cooled and used to pierce and cut thick metal sheets, e.g. 1 inch thick mild steel plate.
Design considerations of these torches include cooling the torch since the arc produces temperatures in excess of 10,000.degree. C. which if not controlled cold destroy the torch, particularly the nozzle. Another consideration is that the arc must be controlled, both to protect the torch itself from the arc and to enhance the quality of the cut being made in a workpiece. An early invention of one of the present applicants described in U.S. Pat. No. 3,641,308 involved the use of a flow of cooling water in the nozzle of a torch to constrict the arc and thereby produce a better quality cut. It has also been found that the cut quality can be greatly enhanced if the plasma is caused to swirl, as by feeding it to the plasma chambers through a swirl ring having a set of off-center holes.
In cutting parts from sheet metal, a cut often begins by piercing the sheet at an interior point. Because the metal is not cut through when the piercing begins, the molten metal cannot run out of the kerf under the force of gravity. It is therefore splashed upwardly onto the torch. This is undesirable because the metal can destabilize the arc, causing it to gouge the nozzle, and it can adhere to the nozzle, which will often lead to double arcing, where the plasma arc flow from the electrode to the nozzle, and then to the workpiece via a conduction path of molten metal. Both gouging and double arcing reduce the nozzle life, or destroy it. It is also important that the resulting cut be smooth, as free of dross as possible, and have a cut angle that is preferably at or near 0.degree., that is, with the "good" side of the kerf having a surface that is perpendicular to the metal sheet itself.
In the past, to control gouging and double arcing due to splattered metal, the solution for high current (200 amperes or more) torches has been to use a multi-piece nozzle with water injection cooling. Typical such nozzles sold by Hypertherm, Inc. are illustrated in schematic form in FIGS. 1A and 1B. Hypertherm Model Nos. HT400 0.099, HT400 0.166 and PAC500 0.187 correspond to FIG. 1a and use a ceramic nozzle face cooled by water. FIG. 2B shows a variation on this design which is sold by Hypertherm, Inc. as its Model PAC500 0.250.
For low current operation, 0-200 amperes, water injection cooling is less practical due to its cost and the energy drain from the plasma by the water cooling. The common commercial solution for low power, air cooled torches was simply to allow the metal to attach the torch part and then replace them. A typical nozzle life such for such a torch operating at 40-50 amperes when piercing and cutting 1/4 inch mild steel is about one hour. There is clearly a cost associated with the replacement parts, the productive time lost during the replacement process, as well as safety considerations that arise whenever a torch is disassembled and reassembled.
Gas cooling of nozzles is also known. It usually involves a dual flow, that is a primary flow of a plasma gas and a secondary flow. They can originate at a common inlet, or separate inlets. The primary flow must be formed by an ionizable gas; the secondary flow is not necessarily ionizable. The primary flow passes through the plasma chamber where it is ionized and exits the torch through its nozzle to form a plasma jet. The secondary gas flows outside the nozzle to form a cold layer of non-ionized gas around the arc. In conventional torches the temperature and velocity of the primary or plasma gas are much higher than those of the secondary gas flow.
While the cutting capabilities of the torch are principally a function of the plasma jet, the secondary flow can be important to cool the torch and to create a protected gaseous environment at the workpiece. FIG. 2A shows a typical use of a secondary flow of gas over the outer surface of a nozzle toward the workpiece. This arrangement is used for low current applications; nozzles of this type are sold by Hypertherm, Inc. as its model Nos. HT40 0.038 and MAX100 0.059. FIG. 2B show another gas cooling arrangement with a ceramic insulating sleeve at the lower end of the nozzle to protect the nozzle against contract against the workpiece. The ceramic, however, is brittle and this arrangement offers no protection of the nozzle during piercing.
U.S. Pat. No. 4,389,559 to Rotolico et al. and U.S. Pat. No. 4,029,930 to Sagara et al. are examples of plasma torches for underwater spraying and welding applications, respectively where a sheath of secondary gas shields the zone where the arc is acting against the surrounding atmosphere, whether air or water. U.S. Pat. No. 4,816,637 to Sanders and Couch discloses a high current underwater cutting torch with an inwardly directed radial flow of air at 0 to 10 scfm in combination with an annular water sheath to create a water-free cutting zone and to sweep away hydrogen gas that would otherwise accumulate under the workpiece.
As noted above, the ability of a plasma torch to pierce is very important in a plasma cutting process. The commonly assigned U.S. Pat. No. 4,861,962 to Sanders and Couch discloses the use of a metallic, electrically floating shield that substantially surrounds the nozzle to block metal splattered on piercing. A secondary gas flow between the shield and the nozzle cools these components. Canted ports upstream introduces a swirl into the secondary flow to help stabilize the arc and improve the cut quality. Bleed ports in the shield also draw off a portion of the cooling flow to allow an increased overall flow for better cooling without destabilizing the arc during cutting. This solution is, however not adequate for high-definition (sometimes termed high-density) torches which have a concentrated arc and require more cooling than a gas can provide. The secondary flow is relatively low in order to maintain the cut quality. The gas functions to cool the torch and to assist in stabilizing the arc.
In dual flow torches, when the primary gas is oxygen or air, the secondary gas is usually air. When the primary gas is nitrogen, the secondary gas is usually carbon dioxide or nitrogen. These combinations produce a suitable plasma jet without an unacceptable level of interference by the secondary gas with the cut. With these secondary gases, the kerf usually exhibits a positive cut angle of 1 to 2 degrees and top and bottom dross. Cut speed and quality are otherwise about the same as if no shield was used.
It is also known to provide different gases, or mixes of gases, for different phases of the cutting operation. For example, Japanese Published Document No. 57-68270 of Hitachi Seisakusho K.K. discloses a preflow of argon during a pilot arc phase, and a switch to hydrogen gas for the cutting, followed by a return to argon after the cutting is terminated. Japanese Published Application No. 61-92782 of Koike Oxygen Industry, Inc. which discloses a nitrogen-oxygen mix as a preflow plasma gas on start up, followed by an oxygen plasma flow. Both of these flows are for the plasma gas, not a secondary gas. This publication teaches that a plasma or primary gas preflow of about 85% nitrogen, 15% oxygen is best to extend electrode life. U.S. Pat. No. 5,017,752 to Severance et al. discloses a flow of a non-oxidizing gas during pilot arc operation which is switched to a pure oxygen flow when the arc transfers. These flows are, again, of primary gas only. Various patents and publications also disclose patterns of gas flow and timing considerations. U.S. Pat. No. 4,195,216 to Beauchamp et al., for example, discloses various modes of operating a plasma-wire welder in a manner that fills the keyhole at the end of the weld by adjusting the wire feed rate in coordination with changes in the gas flow and the arc current.
Applicants are not aware of a torch where an extremely high velocity flow of a secondary gas is used as a gas shield to protect the nozzle and other torch components adjacent the workpiece against splattered molten metal on piercing. Heretofore the lack of uniformity of the flow and flow hysteresis have made the direct interaction of a high velocity gas flow with the plasma jet a situation to be avoided. Applicants are also not aware of the use of a mixture of gases as a secondary gas flow in order to speed the cut and/or increase the cut quality adjustably through a change in the mix of gases forming the secondary gas. In particular applicants are not aware of any secondary gas flow using a mixture of nitrogen and oxygen where the ratio of gases in the mixture is opposite to that of air. Applicants are also not aware of a high definition plasma arc torch that uses a gas shield, this mixture of secondary gases, or flow controls that allow sudden, precise and large changes in the gas flow rates through the torch.
It is therefore a principal object of this invention to provide a plasma arc torch and method of operation that protects the torch against gouging and double arcing during piercing.
Another principal object of this invention is to provide a plasma arc torch and method of operation which increases cutting speed and produces a kerf of enhanced cut quality.
A further object of this invention is to provide the foregoing advantages for a high-definition torch.
Another object is to provide the foregoing advantages, including a cut that has a smooth side surface, a good cut angle, and is substantially free of top dross.
Still another object is to provide the foregoing advantages and also the ability to adjust the cutting operation to adapt to different materials and cutting requirements depending on the application without any changes in equipment.